In recent years, Focused Ion Beam (FIB) apparatuses are used for producing thin film samples for (Scanning) Transmission Electron Microscopes ((S)TEMs). When thin film samples are produced for failure analysis of semiconductor devices in particular, it is desired in most cases to stop FIB processing in a specific cross section. Accordingly, it is general to use FIB-SEM apparatuses which allow observation of a FIB-processed cross section with use of a Scanning Electron Microscope (SEM). In these apparatuses, image observation is generally carried out by using as signals secondary electrons which reflect surface information and irregularity information on a sample.
However, during ion beam irradiation, a large quantity of secondary electrons are generated from a sample by an ion beam, and these secondary electrons overlap with secondary electrons generated by an electron beam as described in Patent Literature 1. Accordingly, during ion beam irradiation, only a SEM image with very low Signal to Noise ratio (S/N ratio) can be acquired.
Examples of a method for obtaining a SEM image during ion beam irradiation include a method in which back-scattered electrons are observed as signals which form an image. The back-scattered electrons have energy higher than that of the secondary electrons. In ion beam irradiation, high-energy electrons are hardly generated. Accordingly, if the back-scattered electrons generated by electron beam irradiation are selected and detected, it becomes possible to acquire a SEM image which is not influenced by the electrons generated by ion beam irradiation.
In the case of the FIB-SEM apparatus, there is a method in which electrons which are transmitted through a sample are observed as signals which form an image. The image (STEM image) formed with transmission electrons is not influenced by the electrons generated by ion beam irradiation. However, acquisition of the STEM image is effective only in the case of a very thin sample which transmits most incident electron beams.
Further, the FIB-SEM apparatus has a function to repeatedly perform processing of a sample by ion beam irradiation and observation of the sample with electron beam irradiation while switching between the processing and the observation.
In the field of SEMs in recent years, there has been conceived a detector (see, for example, Patent Literature 2) which selectively detects secondary electrons (to be referred to as tertiary electrons (SE3) in this specification) discharged when back-scattered electrons induced when a sample is irradiated with an electron beam collide with structures present inside a sample chamber. It is to be noted that the tertiary electrons (SE3) are hardly detected in the case where an Everhardt Thornley Detector, which is a typical secondary electron detector, is used.